1. Field of the Invention
The invention relates generally to the field of production of hydrocarbons from wellbores. More specifically, the present invention relates to a method and apparatus to evaluate the integrity of bonds that adhere wellbore casing to a wellbore.
2. Description of Related Art
Hydrocarbon producing wellbores 2 are drilled from the surface 16 into a subterranean formation 17 containing hydrocarbons entrained therein. Set within the wellbore 2 is casing 4 bonded to the inner surface of the wellbore 2. The casing is bonded within the wellbore 2 by adding cement 6 within the annulus formed between the outer diameter of the casing 4 and the inner diameter of the wellbore 2. The resulting cement bond not only adheres the casing 4 within the wellbore 2, but also serves to isolate adjacent zones (Z1 and Z2) within the formation 17 from one another. Isolating adjacent zones can be important when one of the zones contains oil or gas and the other zone includes a non-hydrocarbon fluid such as water. Should the cement 6 surrounding the casing 4 be defective and fail to provide isolation of the adjacent zones, water or other undesirable fluid can migrate into the hydrocarbon producing zone thus diluting or contaminating the hydrocarbons within the producing zone.
To detect possible defective cement bonds, downhole tools 8 have been developed for analyzing the integrity of the cement 6 bonding the casing 4 to the wellbore 2. These downhole tools 8 can be disposed within the wellbore 2 on a wireline 10 that is connected to a surface truck 14 via a pulley system 12. Typically, transducers 18 are disposed on the outer surface of the tool 8 capable of emitting acoustic waves into the casing 4 and recording the attenuation of the acoustic waves as they travel, or propagate, across the surface of the casing 4. The transducers 18 can either only transmit and receive, or can include those capable of transmitting acoustic signals and receiving a corresponding acoustic signal propagating along the casing. By analyzing the propagation velocity and attenuation of the received acoustic wave, the efficacy and integrity of the cement bond can be evaluated. As is known, pads 19 can be attached to the outer surface of the downhole tool 8 that provide a pedestal on which the transducers 18 can be mounted.
The amount of attenuation however can depend on how the acoustic wave is polarized and coupling condition between the casing 4 and the cement 6 bonding the casing 4 to the wellbore 2. Typical downhole tools 6 having acoustic wave transducers 18 generate acoustic waves that are polarized perpendicular to the surface of the casing 4. Such waves are referred to as compression/shear or P-SV waves since the particle motion direction of either compressional (P) or shear (S) component of the acoustic wave is in a vertical (V) plane perpendicular to the casing 4. The attenuation of the acoustic wave as it propagates along the surface of the casing 4 varies in response to the condition of the cement bond and also in response to the type of cement 6 disposed between the casing 4 and the formation 17. More specifically, as the acoustic wave propagates along the length of the casing 4, the wave loses, or leaks, energy into the formation 17 through the cement bond—it is this energy loss that produces the attenuation of the acoustic wave.
Conversely, when the casing 4 is not bonded, a condition also referred to as “free pipe”, fluid from the formation 17 surrounds the casing 4 instead of cement 6. The fluid behind the casing 4 does not provide for shear coupling between the casing 4 and the formation 17. Loss of shear coupling significantly reduces the compressional coupling between the casing 4 and the formation 17. This result occurs since fluid has no shear modulus as well as a much lower bulk modulus in relation to cement. Because of these physical characteristics of fluid, the entire SV component of the P-SV wave and a large portion of the P component of the P-SV wave do not propagate outside of the casing 4 and thus experience a much reduced attenuation.
Reduced attenuation of an acoustic wave is not limited to situations where the casing 4 is surrounded by fluid, but the presence of some cements can also significantly reduce acoustic wave attenuation. For example, light weight cement (LWC), or cement having a density less than approximately 12 lbs/gal can reduce acoustic wave attenuation. Light weight cement has a shear modulus, thus light weight cement can maintain shear coupling between the casing 4 and the formation 17. However, the density of light weight cement is not substantially greater than the density of many fluids (such as water), thus the attenuation of some acoustic waves, especially P-SV waves, is diminished when encountering casing 4 surrounded by a light weight cement. The result of this reduced attenuation is a decreased ability to detect the difference between fluid and light weight cement as well as a diminished capacity to detect poor cement bonds in light weight cement.
In spite of recent advances in the development of casing bond interrogation devices, room for improving the accuracy and preciseness of these devices still exists.